1. Field of the Invention.
The invention relates in general to selective deposition modeling, and in particular to a method of dispensing a build material to form a three-dimensional object having enhanced dimensional accuracy. The enhanced dimensional accuracy of the three-dimensional objects formed make them useful as sacrificial patterns in investment casting processes or as dimensionally improved rapid prototypes.
2. Description of the Prior Art.
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies are generally called Solid Freeform Fabrication techniques, and are herein referred to as “SFF.” Some SFF techniques include stereolithography, selective deposition modeling, laminated object manufacturing, selective phase area deposition, multi-phase jet solidification, ballistic particle manufacturing, fused deposition modeling, particle deposition, laser sintering, and the like. Generally in SFF techniques, complex parts are produced from a modeling material in an additive fashion as opposed to conventional fabrication techniques, which are generally subtractive in nature. For example, in most conventional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part. SFF technologies typically utilize a computer graphic representation of a part and a supply of a building material to fabricate the part in successive layers. SFF technologies have many advantages over conventional manufacturing methods. For instance, SFF technologies dramatically shorten the time to develop prototype parts and can produce limited numbers of parts in rapid manufacturing processes. They also eliminate the need for complex tooling and machining associated with conventional subtractive manufacturing methods, including the need to create molds for custom applications. In addition, customized objects can be directly produced from computer graphic data in SFF techniques.
Generally, in most SFF techniques, structures are formed in a layer by layer manner by solidifying or curing successive layers of a build material. For example, in stereolithography a tightly focused beam of energy, typically in the ultraviolet radiation band, is scanned across a layer of a liquid photopolymer resin to selectively cure the resin to form a structure. In Selective Deposition Modeling, herein referred to as “SDM,” a phase change build material is jetted or dropped in discrete droplets, or extruded through a nozzle, to solidify on contact with a build platform or previous layer of solidified material in order to build up a three-dimensional object in a layerwise fashion. Other synonymous names for SDM used in this new industry are solid object imaging, deposition modeling, multi-jet modeling, three-dimensional printing, thermal stereolithography, and the like. Often, a thermoplastic material having a low-melting point is used as the solid modeling material, which is delivered through a jetting system such as an extruder or print head. One type of SDM process which extrudes a thermoplastic material is described in, for example, U.S. Pat. No. 5,866,058 to Batchelder et al. One type of SDM process utilizing a printer to selectively dispense a liquid binder on a layer of powder is described in, for example, U.S. Pat. No. 6,007,318 to Russell, et. al. One type of SDM process utilizing ink jet print heads is described in, for example, U.S. Pat. No. 5,555,176 to Menhenneft et al. Some thermoplastic build materials used in SDM are available and sold under the names ThermoJet® 2000 and ThermoJet® 88 by 3D Systems, Inc. of Valencia, Calif. Also, some formulations for thermoplastic phase change build materials are disclosed in U.S. Pat. No. 6,132,665 to Bui et al. which is herein incorporated by reference as set forth in full.
SDM systems have certain advantages over other SFF systems such as stereolithography. One significant advantage of SDM systems is that they are significantly less expensive than stereolithography systems. This is generally due to the use of relatively low cost dispensing devices employed in SDM systems such as ink jet print heads, instead of the expensive lasers and scanning components used in stereolithography systems.
However, SDM systems have some disadvantages. Because most materials used in SDM inherently shrink as they solidify, the solid three-dimensional objects are subjected to non-linear distortion. Undesirably, this non-linear distortion can cause, for example, flat planar surfaces to bow or distort as the object cools down to ambient temperatures. In contrast to linear shrinkage, with occurs in all three dimensions and can be compensated for with shrink compensation factors, this non-linear distortion generally cannot be compensated for since it is dependent on the geometry of the object and the inherently non-uniform cooling rate of the object. Thus, solid objects formed by SDM are not well suited in applications where high dimensional accuracy is needed; for example, such as in investment casting where the object is utilized as the sacrificial pattern for forming the final cast part.
Previous expedients in stereolithography have developed build styles which produce hollow structures exhibiting high dimensional accuracy for use in investment casting. An example of one such technique is the Quickcast™ build style used by 3D Systems, Inc., which produces three-dimensional objects from a cured photopolymer resin. These three-dimensional objects have a skin and a honeycomb-like internal structure as disclosed in U.S. Pat. No. 6,110,602 to Dickens et al. Importantly, the honeycomb-like internal structure comprises a plurality of spaced-apart plates connected together at their corners or edges so that the structure will buckle or give during the investment casting process. This is necessary so that the object will not expand and crack the ceramic slurry when the slurry is cured. Often temperatures of around 1300° F. are needed to cure the slurry and to burn out the cured photopolymer material which causes thermal expansion in the object that is compensated for by the buckling honeycomb-like build style. When working with phase change materials in SDM, providing a staggered honeycomb-like structure to provide give for thermal expansion in the object is not necessary since temperatures of around only 200° F. are needed to melt and remove the material in the investment casting process. In addition, in the QuickCast build style all the hollow cavities are interconnected to allow for drainage of un-solidified liquid photopolymer after the object is formed. However, this interconnected structure is not necessary when working with non-curable phase change materials in SDM since there is no un-solidified liquid present after the object is formed. Thus, this build style is generally not suitable for use in SDM applications utilizing a single phase change material.
Another expedient in stereolithography build styles which produce hollow structures for use in investment casting is disclosed in U.S. Pat. No. 6,309,581 to Gervasi. In this build style an internal lattice structure of the cured photopolymer material comprises a plurality of tetrahedron structures, each tetrahedron having four legs extending in different directions from a single point. This structure also provides the necessary give in the object so that during the investment casting process the structure will not crack the ceramic slurry. However, forming the tetrahedron structure by SDM is not possible unless a support structure is formed to support the three non-vertically oriented legs of each tetrahedron which also adds unnecessary complexity to the build process. Thus, this build style is also generally not suitable for use in SDM.
Thus, there is a need to develop a method for forming three-dimensional objects by SDM with enhanced dimensional accuracy. These and other difficulties of the prior art have been overcome according to the present invention.